Controlled atmosphere processing



May 13, 1969 w. R. Ki-:OUGH

CONTROLLED ATMOSPHERE PROCESSING Filed May 9, 1966 w e n ,c .m .m im ,mm. .5 M m I o 5 0 mi. U m .w n v w .mlvm mlv/n 'vm M w. .m 2 Mu w m We.mm u .r Mw im P Ot O H om Us H V I United States Fatent 3,444,008CONTROLLED ATMOSPHERE PROCESSING William R. Keough, Birmingham, Mich.,assignor of fortyve percent to Multifastener Company, Detroit, Mich., apartnership Filed May 9, 1966, Ser. No. 548,554 Int. Cl. C21d 7/14,1/60, 9/52 U.S. Cl. 14S-12.4 9 Claims ABSTRACT F THE DISCLUSURE A methodof heat treating steel objects which includes heating the steel to anaustenitic state, quenching the object in liquid to reduce thetemperature of the object to a lower temperature at which the steelremains austenitic for a substantial time and then transforms to anothercrystalline form with the quenching being done suiiiciently fast toavoid transformation of the steel to another crystalline form, workingthe object at the lower temperature in said liquid, maintaining theobject at the lower temperature while the steel thereof transforms tothe other crystalline form, and ultimately cooling the object to roomtemperature.

This invention relates to methods of heat treating steel, and moreparticularly to a heat treatment involving quenching of austenitic steeland working of the quenched steel while it remains in an austeniticstage so as to greatly increase the tensile strength and ductility ofthe steel without introducing internal stresses.

There are many applications for steel of high tensile strength and greatductility. Considering only steel wire as an example, such propertiesare desired for bridge cables, elevator cables, cranes, hoists, andother applications where heavy loads are carried by the wire. Hotworking and cold working of steel are well known ways of increasing itstensile strength. In the case of hot working, the grains of the steelare round and this is not the optimum condition for high tensilestrength. Steel having elongated grains can be obtained by cold working,and such elongation of the grains increases the tensile strength of thesteel. However, cold working introduces internal stresses in thematerial which detract from its tensile strength and tends to make thesteel more brittle. Furthermore, cold working elongates only the grainsnear the surface of the material.

The method of heat treating steel according to the present inventionelongates the grains of the steel throughout the material and results insteel having high tensile strength and also great ductility. Inachieving the elongation of the grains of the steel, internal stressesare not introduced into the steel, and thus the method of the inventionis superior to cold working.

Briey, the method of the invention includes the steps of:

1) Heating the steel to a temperature at which it becomes austenitic;

(2) Cooling the steel sufficiently fast to avoid transformation oftheaustenite to a lower temperature at which the austenite transforms tobainite;

(3) Working the steel at the lower temperature while the steel remainsaustenitic;

(4) Maintaining the steel at the lower temperature while it transformsto bainite; and

(5) Ultimately cooling the steel to room tempearture.

The rapid cooling of the steel can be done by quenching it in a liquidmaintained at the temperature for transformation to bainite. The lattertemperature is above the temperature at which martensite starts to form.The working can be done in the case of wire stock by rolling the wirebetween dies to reduce its cross-sectional area. The

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transformation of the steel from austenite to bainite after quenching isan isothermal process which in itself hardens and strengthens the steel,and the working of the steel at the transformation temperaturecooperates with the quenching and transforming steps to achieve a stillhigher tensile strength and greater ductility.

It is, therefore, an object of the present invention to provide a methodo-f heat treating steel which results in steel having high tensilestrength and great ductility.

Another object of the invention is to combine working of steel objectswith quenching and isothermal transformation steps so as to producesteel of high tensile strength and great ductility Without introducinginternal stresses into the steel.

A further object of the invention is to provide a method of heattreating steel wherein austenitic steel is quenched to a tempearture forisothermal transformation to bainite and is worked while the steelremains in the austenitic stage to obtain a final product characterizedby elongated grains free of internal stresses.

Other objects of this invention will appear in the following descriptionand appended claims, reference being had to the accompanying drawingsforming a part of this specification wherein like reference charactersdesignate corresponding parts in the several views.

On the drawings:

FIGURE 1 is an isothermal transformation diagram illustrating the heattreatment of the invention;

FIGURE 2 is a ilow diagram of the heat treatment of the invention;

FIG. 3 represents a photomicrograph of hot-worked steel showing roundgrains; and

FIGURE 4 represents a photomicrograph of steel treated by the method ofthe invention and having elongated grains.

Before explaining the present invention in detail, it is to beunderstood that the invention is not limited in its application to thedetails of construction and arrangement of parts illustrated in theaccompanying drawings, since the invention is capable of otherembodiments and of being practiced or carried out in various ways. Also,it is to be understood that the phraseology or terminology employedherein is for the purpose of description and not of limitation.

As shown on the drawings:

The method of heat treating steel according to the invention will bedescribed with reference to the isothermal transformation diagram shownin FIGURE 1. The diagram gives only one example of austenitetransformation as it occurs in a particular steel, but it is believedthat this example will serve to illustrate the factors which are takeninto consideration in planning a heat treatment in accordance with theinvention.

In the diagram of FIGURE l, time is plotted in seconds on a log scalealong the abscissa axis, and temperature is plotted in degrees F. alongthe ordinate axis. Two temperatures labeled AS and Af are represented byhorizontal lines near the top of the diagram.

The As temperature is that temperature at which a trace amount ofaustenite forms in the ferrite matrix and does not increase perceptiblyin amount when the holding time at As is doubled.

The Af temperature denotes the maximum temperature at which a barelydetectable vamount of ferrite can exist in the steel. These temperaturesare the limiting or ceiling temperatures for isothermal transformation.Austenite is a solid solution of Fe3C in gamma iron produced by verydrastic quenching of carbon steel. It is soft, nonmagnetic and is thedensest constituent of any steel.

Near the bottom of the diagram, there is a horizontal dashed line,labeled Ms, which indicates the temperature at which martensite startsto form on quenching from the austenitizing temperature.

There are three curves 10, 11 and 12 in the diagram. Curve 10, which isthe first curve from the left, is called the beginning line andrepresents the time required for a measurable amount of austenite totransform. The second curve 11 represents the beginning oftransformation to ferrite-carbide aggregate. The curve 12 farthesttoward the right represents the time required at each temperature forthe last trace of austenite to transform. Each lield on the diagramabove Ms is labeled to indicate the phases observed in specimensaustenitized and then quenched and held isothermally within thetime-temperature limits of each iield. In the diagram, A is austenite, Fis ferrite, C is carbide, and M is martensite. In general, acicularaggregates classied as bainite form from the nose temperature down toMs. Bainite is a dark etching acicular aggregate obtained by theisothermal transformation of austenite between 400 F. and 1000 F. Thistemperature range is just above the temperature to which the steel mustbe quenched to form martensite.

The line drawn through points W, X, Y and Z is the cooling curve(generally designated 14) for a heat treatment in accordance with theinvention. The irst step of that method is to heat the steel to atemperature at which it becomes austenitic throughout. For plain carbonsteels, this temperature may abe in the range from about 1400 to about1700 F. The cooling curve 14 of the diagram is for a 1060 steelcontaining about 0.64 percent carbon and 1.13 percent manganese, and theaustenitizing temperature is about 1500 F.

After the steel has been austenitized, it is cooled fast enough to avoidany transformation of the austenite to a lower temperature (representedby point X on the diagram) at which the austenite transforms to bainite.This portion of the cooling curve is represented by the line betweenpoints W and X in the diagram of FIGURE 1, and it may be seen that thisline lies to the left of the beginning line for transformation, curve10, such that there is no transformation of the austenite during therapid cooling step.

The rate of cooling will vary from composition to composition, but thecooling line 14 must be kept to the left of the nose of the beginningline in order to avoid transformation during cooling of the steel.

The rapid cooling may be carried out by quenching the steel in a saltmedium which is maintained in the lower temperature X for transformationof the austenite to bainite.

The portion of the cooling curve passing through points X, Y and Z ishorizontal, indicating that the steel is held at a constant temperaturewhile it transforms. This transformation temperature is above the Msline so martensite does not form. In the heat treatment of theinvention, the rapid cooling is always terminated at a temperature abovethat at which martensite starts to form. The steel is not merely held atthis transformation temperature, but rather it is worked while the steelremains austenitic and before the transformation to bainite begins. Theportion of curve 14 where working takes place is between points X and Yas indicated on a diagram` of FIGURE l, and it is clear that the workingis done before transformation begins. The transformation begins at pointY and is complete at point Z. Thus, two steps take place between pointsX and Z of the diagram. The steel is worked at the transformationtemperature (between points X and Y) to elongate the grains of thesteel, and thereby increase its tensile strength and ductility, and thesteel is maintained (between points Y and Z) at a constanttransformation temperature while it transforms to bainite. After thesteel has transformed, it may be cooled to room temperature.

The working step is carried out while the steel is in a salt quenchingbath. In the case where steel wire is treated by the method of theinvention, the wire may be passed through a reduction die in the saltbath which may be .4 a series of rollers mounted with their axes offsetat to each other. The rollers achieve an overall reduction in thediameter of the wire. The reduction may be about 30 to 40 percent, orwhatever is needed to obtain elongation of the grains of the steelthroughout the structure.

FIGURE 4 shows the type of elongated grain structure which is desired,and for purposes of comparison FIGURE 3 illustrates a sample havinground grains, such as might be obtained by hot working of steel. It maybe seen in FIGURE 4 that the elongation of the grains takes placethroughout the material rather than just at the outside of the material,as is the case where steel is cold worked.

The following examples give the times and temperatures for the variousportions of the heat treatment for 1050 and 1060 steels.

1050 MATERIAL The heat treatment of the invention may be applied tosteels of varying compositions and it is particularly advantageous asapplied to plain carbon steels, especially in the form of steel wire.The steel may contain a minor amount of an alloying element such assilicon, chromium or manganese or mixtures thereof. The invention is notnecessarily restricted to treatment of Wire. For example, sheet materialand rods might Well be treated by the method of the invention.

'I'he invention provides a method of heat treating steel -in whichworking is accomplished while an austenitized and quenched steel is heldat a transformation temperature such that the nal steel is characterizedby elongated grains throughout its microstructure. This imparts a highdegree of tensile strength and ductility to the material and does notintroduce internal stresses.

I claim:

1. A method of heat treating objects made of steel comprising the stepsof heating such an object to an elevated temperature at which the steelthereof becomes austenitic, quenching said object in a salt medium tocool said object to a lower temperature sufliciently fast to avoidtransformation of the austenite to another crystalline form, said lowertemperature being one at which the steel remains austenitic for asubstantial time and then transforms to another crystalline form,working said object at said lower temperature while said object is insaid salt medium and while said steel remains austenitic in order toincrease the tensile strength and ductility of the final steel afterheat treatment, maintaining said object at said lower temperature whilethe steel thereof transforms to said other crystalline form, andultimately cooling said object to room temperature.

2. A method as defined in claim 1, in which said other crystalline formis bainite.

3. A method as deiined in claim 1, in which said steel object is wireand in which said working step is carried out by rolling said steel wireto achieve an overall reduction in the diameter of said wire.

4. A method as defined in claim 1, in which said Wonking step includesrolling said steel object to decrease the 5 cross-sectional area of saidobject and increase the length thereof.

5. A method of heat treating objects made of steel comprising the stepsof heating such an object to an elevated temperature at which the steelthereof becomes austenitic, then quenching said object in a liquidmedium to cool the same sufliciently fast to avoid transformation of theaustenite and to a lower temperature at which the austentite transformsto bainite, working said object at said lower temperature and in saidliquid medium before the transformation :to bainite begins, holding saidobject at said lower temperature and in said liquid medium While thesteel thereof transforms to bainite, and ultimately cooling said objectto room temperature.

6. The method of claim 5 in which said steel is a plain carbon steel.

7. A method of heat treating objects made of steel comprising the stepsof heating such an object to an elevated temperature at which the steelthereof becomes austenitic, quenching said object in a liquid which ismaintained at a temperature at which the steel remains austenitic for asubstantial time and then transforms isothermally to bainite, saidquenching being accomplished in a period of time less than the timerequired for transformation of the austenite to another form, workingsaid object at said lower `temperature in said liquid while said steelremains austenitic and before transformation thereof begins in order toincrease the tensile strength and ductility of the steel, maintainingsaid object at the ternperature of said liquid while the steel thereoftransforms to bainite, and ultimately cooling said object to roomtemperature.

8. The method of claim 7 in which said object is reduced incross-sectional area by said working step.

9. The method of claim 3 in which said steel object is wire and saidworking step is carried out by rolling the wire to reduce the crosssectional area thereof.

References Cited UNITED STATES PATENTS 2,435,511 2/1948 Rice 14S-12.4

L. DEWAYNE RUTLEDGE, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

